The goal of a triathlon is to provide a challenge to the athlete, but this comes with inherent risks. The drowning rate of triathlon participants has increased exponentially over the years in open water triathlon venues. This study sought to understand the decision-making process of experienced lifeguards in the unique conditions of the open water venue in order to inform subsequent training. Application of the naturalistic decision-making framework provided a means to develop an understanding of decision-making in this complex, uncertain environment. A basic interpretive study was undertaken to address the research question: How do experienced lifeguards make decisions during open water triathlons? Nine themes emerged that described how lifeguards make decisions: (a) lifeguards use critical information provided in prerace activities, (b) lifeguards apply constant awareness to scene management, (c) lifeguards use continual communication, (d) lifeguards use cues taken from swimmers, (e) lifeguards use pattern recognition of common problems, (f) lifeguards use rapid triage assessment, (g) lifeguards make automatic rescue responses from repeated practice, (h) lifeguards' sense of duty and empathy guides their concern for swimmers, and (i) lifeguards receive support and relief from emergency medical services. The nine themes indicated that the decision-making process of guarding open water triathlons is multifaceted and complex; specific environmental conditions impact the decision-making process, and relevant training provides a strong foundation but is inconsistent. Three conclusions were drawn from the study: (a) the situation awareness and recognition-primed decision-making models explain the major cognitive processes involved in lifeguards' decision-making in open water triathlons, (b) the lifeguard decision-making process in an open water triathlon includes additional critical external factors not part of the naturalistic decision-making models, and (c) situation awareness is a fundamental part of the lifeguard decision-making process and training for an open water triathlon. The study offers recommendations related to theory, practice, and future research. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]